Once through boilers are boilers in which the operating fluid such as water is vaporized and superheated in one passage through the boiler, without a separation step of steam from water.
FIG. 1 shows a typical power plant set up with a once-through Boiler 1 and a steam turbine 2.
The boiler 1 has a water supply 4 connected to an economizer 5 in turn connected to an evaporator 6 in turn connected to a separator 7 (for forced circulation operation). A line 8 with a pump 9, is connected between the separator 7 and the water supply 4, and a line 10 is connected between the separator 7 and a superheater 11. A line 13 is provided between the superheater 11 and a high-pressure turbine 14; the line 13 is provided with a control valve 15. Usually also a closing valve (for safety reasons) is provided, the closing valve is not shown in the drawings; in addition, the control valve and the closing valve can be both embedded in one body.
A line 16 is provided between the high-pressure turbine 14 and a reheater 17 and a line 18 connects the reheater 17 to a medium-pressure and/or low-pressure turbine 19. Downstream of the turbine 19 a condenser is provided (not shown). In other cases, downstream of the turbine 19 another steam consumer might be installed.
The boiler 1 does not have valves between the different boiler sections (economizer 5, evaporator 6, superheater 11, reheater 17); it is anyhow clear that the method can also be implemented on boilers (1) that are equipped with these valves.
Usually, the water supply 4 is connected to a preheater (condenser), preferably without mixing of the fluids involved, for pre-heating the water supplied to the economizer 5. The preheater 21 is connected via an extraction line 22 having a regulation valve 23 to the line 16, for supplying steam from the line 16 into the preheater 21; at the preheater 21 the steam is condensed (but the steam is not mixed with the water) to preheat the water supplied to the economizer 5.
Normal Operation
During normal operation, water is heated at the preheater 21 by condensing steam from the line 16. The steam is not mixed with the water (i.e. at the condenser 21 steam and water are maintained separated from each other). The heated water is then supplied via the water supply 4 to the economizer 5 and evaporator 6; at the evaporator 6 water is completely evaporated, such that from the evaporator 6 steam, usually superheated steam, is supplied into the separator 7.
During normal operation, the forced circulation operation is out of service and the circulation pump 9 is switched off.
The steam from the separator 7 is thus further superheated in the superheater 11 (shown in the drawing in two sections with a spray water attemperator in between) and then expanded in the high-pressure turbine 14. The steam discharged from the high-pressure turbine 14 is then reheated in the reheater 17 and further expanded in the turbine 19 and is then forwarded to the condenser of the steam cycle (not shown in FIG. 1). The condensed fluid is returned to the boiler.
Low Load Operation
When once through boilers are being operated at low load, the water supplied via the water supply 4 is reduced, in order to reduce the steam mass flow generated at the boiler 1 and supplied to the high-pressure turbine 14 and medium/low-pressure turbine 19.
Anyway, below a certain load, the water flow supplied via the water supply 4 becomes so small, that stable operation of the boiler 1 cannot be guaranteed any longer. For example, when the water flow becomes too low, at the evaporator and/or economizer high tube wall temperatures and/or temperature differences in tube walls and between tubes may occur, due to problems such as departure from nuclear boiling and/or static instability and/or too low static stability of the water flow; other problems that may occur are different flows in different tubes and/or flow dynamic instability, fluctuating flows and/or pressures and/or temperatures.
Too small a flow via the water supply 4 through the economizer and evaporator shall thus be avoided.
Traditionally, in order to avoid operation with a too small water/steam flow through the economizer and evaporator, the boiler is switched from once-through operation (described above) to forced circulation operation.
In forced circulation operation, the water supply 4 feeds a higher water mass flow to the economizer 5 and evaporator 6 than the water mass flow that can be evaporated with the currently supplied fuel required for the current load.
The evaporator 6 thus only partially evaporates the water and provides the separator 7 with a mixture of water and steam.
The water is collected at the bottom of the separator 7 and is fed back by the pump 9 upstream of the evaporator 6 or economizer 5 (e.g. FIG. 1 shown supply via line 8 upstream of the economizer 5).
This way it is possible to maintain a water/steam flow through the evaporator 6 and economizer 5 or only the evaporator 6 greater than the water/steam flow needed at that load, such that the water mass flow passing through the economizer 5 and/or evaporator 6 is high enough to guarantee reliable operation without damages of the boiler.
Nevertheless, during forced circulation operation, the steam collected at the separator 7 is not superheated because the water/steam system is in the wet steam area.
Since the steam in the separator 7 is not superheated, as it would be the case in once through operation, the steam temperature at the outlet of the evaporator 6 drops significantly compared to once-through operation, leading to a reduced steam temperature also at the outlet of the superheater 11.
These temperature excursions are detrimental to the material life time, it is therefore desirable to maintain the once-through operation for the boiler also at low load or at least reducing the limit load where switching over to forced circulation operation is necessary.
In addition, when at low load the steam temperature at the superheater outlet is kept closer to the nominal temperature, re-ramp rates could be adjusted to much higher gradients. When at lower load forced circulation is not required, the time consuming switch over process from forced circulation to once-through operation is also no longer required. With this switch over process being made redundant, the time period that is necessary to ramp the unit back up to nominal load coming from low load is reduced.